1. Field of the Invention
The present invention relates to an corrosion-resistant system and method for a plasma etching apparatus in which a relatively thick portion(s) on a surface of an object to be etched are locally etched.
2. Description of the Related Art
FIG. 5 is a cross section showing an example of a known plasma etching apparatus, and FIG. 6 is a diagram showing the state of oscillation of a micro wave.
The plasma etching apparatus performs plasma-discharge using a micro wave. A mixed gas containing a halogen-based gas such as CF.sub.4 (carbon tetrafluoride) is supplied to a quartz discharge tube 110 which is mounted on an upper surface of a chamber 100. As shown in FIG. 6, the micro wave M of about 500W is generated or oscillated continuously by a micro wave oscillator 120 toward a wave guide 121, so that the mixed gas in the quartz discharge tube 110 is made into a plasma state, thus producing active species such as fluorine radicals which contribute to etching a silicon wafer W.
On the other hand, the silicon wafer W is fixedly mounted on a stage 101 which is driven to move in an X-Y direction (i.e., the right and left direction as well as the front and rear direction of the sheet of FIG. 5) by means of an X-Y drive mechanism 130.
Specifically, a movable stand 132 carrying thereon a stage 101 is driven to move in an X-axis direction by means of a drive motor 131 and in a Y-axis direction by means of a drive motor 133 mounted on the movable stand 132.
With this construction, the active species such as fluorine radicals generated by the plasma discharge is jetted from an ejection opening 110a of a quartz discharge tube 110 onto a silicon wafer W. At the same time, the X-Y drive mechanism 130 moves a relatively thick portion of the silicon wafer W (i.e., a portion which forms a surface of the silicon wafer W is formed, and which is relatively thicker than a specified thickness) right under the ejection opening 110a of the quartz discharge tube 110 so that the relatively thick portion can be partially or locally etched.
Here, it is to be noted that during such partial or localized etching, part of the active species gas G jetted from the ejection opening 110a might diffuse to etch an inner wall of the chamber 100 as well as the X-Y drive mechanism 130.
To avoid this, the plasma etching apparatus employs an corrosion-resistant technique.
That is, the inner wall of the chamber 100 and the X-Y drive mechanism 130 are subjected to corrosion-resistant coating so that they can be prevented from being etched by the active species gas G. Furthermore, even if those portions such as threaded portions, rails, bearings of the X-Y drive mechanism 130, rotation shafts of the drive motors 131, 133 and so on, which are in sliding contact with other elements, are applied with corrosion-resistant coatings, such corrosion-resistant coatings would be liable to be peeled off during a long period of use. Thus, those contacting and sliding portions are coated with a corrosion-resistant oil which does not flake off due to repeated sliding actions.
The above-mentioned plasma etching apparatus and corrosion-resistant technique have the following problems.
A first problem is that since the plasma etching apparatus is constructed such that a micro wave M is oscillated or generated continuously so as to produce plasma discharge, as illustrated in FIG. 6, the effective period or life time of the quartz discharge tube 110 is short and the etching rate with respect to the silicon wafer W is low, and the silicon wafer W during etching is liable to be contaminated.
FIG. 7 is a cross section showing the state of corrosion of the quartz discharge tube 110.
If a mixed gas containing a CF.sub.4 (carbon tetrafluoride) gas for instance is plasma-discharged, there will be generated an active species gas G which contains CF.sub.3 radicals, F radicals, CF.sub.3 cations or positive ions, F anions or negative ions, etc. These radials, positive and negative ions contribute to the localized etching of the silicon wafer W.
However, when plasma discharging is continuously conducted, the quartz discharge tube 110 continuously absorbs the micro wave, resulting in a rapid rise of the heating temperature of the quartz discharge tube 110. As a consequence, the reaction of the active species gas G and a SiO.sub.2 (silicon dioxide) component of the quartz discharge tube 110 is promoted, as shown in FIG. 7, so that the inner wall of the quartz discharge tube 110 is subjected to corrosion, thus forming a hole or holes through the quartz discharge tube 110 in a relatively short period of time.
Furthermore, when the corrosion of the quartz discharge tube 110 has been made, the active species gas G reacts with the quartz discharge tube 110 to turn into a SiF.sub.4 (silicon tetrafluoride) gas, thereby reducing the density of the active species gas G ejected to the silicon wafer W to lower the etching rate of the silicon wafer W.
In addition, during the etching of the quartz discharge tube 110, there are generated particles of impurities contained in the quartz discharge tube 110 itself. These particles might be jetted to the surface of the silicon wafer W, thus contaminating the silicon wafer W.
A second problem is that the above-mentioned corrosion-resistant technique does not provide a satisfactory corrosion resistant effect.
Specifically, it is impossible to completely provide all the exposed portions of the chamber 100, the X-Y drive mechanism 130 and the like with corrosion-resistant coatings. Especially, the X-Y drive mechanism 130 is constructed of various members assembled, so it is impossible to coat every component member with a corrosion-resistant oil.
Further, the corrosion-resistant oil applied to the sliding portions gradually evaporates so that the base portions underlying the corrosion-resistant coatings are exposed during a long period of use. In order to avoid this, it is necessary to disassemble the chamber 100 and the X-Y drive mechanism 130 regularly or at a predetermined interval and re-coat them with the corrosion-resistant oil.